In order to understand the spectral
distribution of infrared radiation from a source we must first understand Planck's Law.
Planck's Law gives us the spectral distribution of radiation from a blackbody source. That
is, a source that emits 100% infrared radiation at a given single temperature. It is
important to understand at this point that in practice, infrared sources are made up of
thousands of "point sources" that are all at different temperatures. Each point
source will have a different spectral distribution and the combination of point sources
will make up the entire spectral distribution. Therefore, we can only approximate the
spectral distribution using an average surface temperature and emissivity value.